1. Field of the Invention
The invention is directed to a roll-coating machine for applying coating colors onto a paper web.
2. Description of Related Technology
One type of known roll-coating machine for applying a uniform coat of coating color(s) onto a paper web is disclosed in DE-OS 36 05 409 (Aug. 27, 1987) and includes (a) a coating roll that rotates in a coating color pool received in a trough that has a coating color inlet, a guide wall in the ascending region of the coating roll with a first overflow edge, and an overflow channel connected to the overflow edge for a first overflow stream, (b) a mating roll that is wound with a paper web and rotates in a direction opposite that of the coating roll and defines inlet and outlet gussets in cooperation with the coating roll, and (c) a machine-wide guide shield that extends into the inlet gusset, defines a flow channel in cooperation with the surface of coating roll, and has a second overflow edge for a second overflow stream on its downstream end.
Problems develop in such roll-coating machines mostly in the wedge-shaped inlet zone between the coating roll and the mating roll, especially at high speeds. This is largely attributable to the effect of air, which may be entrained into the inlet gusset during rotation of the mating roll together with the paper web. When this happens the air mixes with the coating color at the site where the color contacts the paper web. An overflow stream of coating color generally forms on the upper edge of the guide shield opposite the running direction of the paper web and opposite the air stream. If the air stream collides with this overflow stream, this leads to partial repulsion of the overflow stream, as well as mixing of air and coating color.
Disturbances of this type also occur if no guide shield is provided and if, as a result, a free surface of coating mass appears between the surface of the coating roll and the wall of the trough.
It would be desirable that the paper web and the coating color form a straight contact line on joining. However, owing to the described effect of entrained air, such contact is often not linear, but rather tongue-shaped. This is true of roll-coating machines with guide shields as described above and therefore of roll-coating machines with a free surface. If persistent air inclusions are retained over the entire coating zone, the paper web is not wetted at all by the coating color at the sites in question and remains uncoated at these sites. This is of course extremely undesirable.
An additional important requirement is that the coating color be fed in sufficient amount to the inlet gusset and thus the paper web, starting from an essential minimum amount to a maximum amount.